<i>Hox</i> genes control homocercal caudal fin development and evolution.

Abstract

Ancient bony fishes had heterocercal tails, like modern sharks and sturgeons, with asymmetric caudal fins and a vertebral column extending into an elongated upper lobe. Teleost fishes, in contrast, developed a homocercal tail characterized by two separate equal-sized fin lobes and the body axis not extending into the caudal fin. A similar heterocercal-to-homocercal transition occurs during teleost ontogeny, although the underlying genetic and developmental mechanisms for either transition remain unresolved. Here, we investigated the role of <i>hox13</i> genes in caudal fin formation as these genes control posterior identity in animals. Analysis of expression profiles of zebrafish <i>hox13</i> paralogs and phenotypes of CRISPR/Cas9-induced mutants showed that double <i>hoxb13a</i> and <i>hoxc13a</i> mutants fail to form a caudal fin. Furthermore, single mutants display heterocercal-like morphologies not seen since Mesozoic fossil teleosteomorphs. Relaxation of functional constraints after the teleost genome duplication may have allowed <i>hox13</i> duplicates to neo- or subfunctionalize, ultimately contributing to the evolution of a homocercal tail in teleost fishes.